Classifications

• • H—ELECTRICITY
  • H03—ELECTRONIC CIRCUITRY
  • H03F—AMPLIFIERS
  • H03F3/00—Amplifiers with only discharge tubes or only semiconductor devices as amplifying elements
  • H03F3/181—Low-frequency amplifiers, e.g. audio preamplifiers
  • H03F3/183—Low-frequency amplifiers, e.g. audio preamplifiers with semiconductor devices only
  • H03F3/185—Low-frequency amplifiers, e.g. audio preamplifiers with semiconductor devices only with field-effect devices
  • H03F3/1855—Low-frequency amplifiers, e.g. audio preamplifiers with semiconductor devices only with field-effect devices with junction-FET devices
• • H—ELECTRICITY
  • H03—ELECTRONIC CIRCUITRY
  • H03F—AMPLIFIERS
  • H03F2200/00—Indexing scheme relating to amplifiers
  • H03F2200/03—Indexing scheme relating to amplifiers the amplifier being designed for audio applications
• • H—ELECTRICITY
  • H03—ELECTRONIC CIRCUITRY
  • H03F—AMPLIFIERS
  • H03F2200/00—Indexing scheme relating to amplifiers
  • H03F2200/372—Noise reduction and elimination in amplifier
• • H—ELECTRICITY
  • H03—ELECTRONIC CIRCUITRY
  • H03F—AMPLIFIERS
  • H03F2200/00—Indexing scheme relating to amplifiers
  • H03F2200/54—Two or more capacitor coupled amplifier stages in cascade
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04R—LOUDSPEAKERS, MICROPHONES, GRAMOPHONE PICK-UPS OR LIKE ACOUSTIC ELECTROMECHANICAL TRANSDUCERS; DEAF-AID SETS; PUBLIC ADDRESS SYSTEMS
  • H04R25/00—Deaf-aid sets, i.e. electro-acoustic or electro-mechanical hearing aids; Electric tinnitus maskers providing an auditory perception
  • H04R25/50—Customised settings for obtaining desired overall acoustical characteristics
  • H04R25/502—Customised settings for obtaining desired overall acoustical characteristics using analog signal processing

Definitions

• This invention relates generally to audio frequency amplification circuits used as the preamplifier stages of microphone transducers. More particularly, the invention relates to circuits employing field effect transistors operating with low voltage sources, suitable for use with electret based condenser microphones.
• the signal-to-noise ratio for electret based condenser microphones has not significantly improved in the past 25 years. During that time there has been a decrease in transducer size, with the signal-to-noise ratio either remaining the same or, in the case of the later smaller transducers, actually declining along with the signal.
• transducers having high source impedance require low noise, high input impedance amplifiers.
• such transducers including for example electret based condenser microphones, have such a high source impedance that the coupling of external, unwanted signals will occur through the electrical leads.
• the high impedance of such transducers requires that the preamplifier have an ultra-high input impedance to prevent signal reduction at the input.
• JFET N channel junction field effect transistor
• U.S. Patent No. 5,097,224 issued on March 17, 1992 to the present applicant and Michael J. Wurtz, discloses a preamplifier circuit with a field effect transistor configured in the follower mode. In this circuit a source resistor comprises the load impedance.
• U.S. Patent No. 4,151,480 issued on April 24, 1979 to Carlson et al discloses an amplification circuit employing, in addition to a field effect transistor connected in follower mode, a second field effect transistor used as a current source in lieu of a source resistor, thus providing a much higher ac load impedance than would a simple resistor with the same dc current flowing through it.
• U.S. Patent No. 3,512,100 issued May 12, 1970 to Burkhard et al discloses a pair of diodes of opposite polarity connected in parallel as the gate bias of a field effect transistor connected in follower mode. Whereas a resistor of large impedance used for gate bias can introduce noise at low frequencies (up to about 1000 Hz), the use of diodes dramatically reduces noise at those frequencies because the diodes act as resistors of very much larger values. However, such diodes can be prone to failure due to electrostatic discharge.
• the present invention recognizes two causes that are basically responsible for the previous lack of progress.
• the first is a misconception of the noise sources in a modern transducer.
• the second misconception is a failure to match transistor design to transducer design.
• the present invention provides a low noise, low input capacitance impedance converter using field effect transistors for the active devices, capable of operation from low power supply voltages as small as a single cell.
• the circuit is suited as the preamplifier for transducers, especially electret based condenser microphones used in hearing aids.
• the preferred embodiments of the invention include two N channel junction JFET follower stages connected by two capacitors, one used for coupling between the stages and one for frequency shaping.
• the first stage affords low input capacitance, improved power supply feedthrough reduction, frequency shaping of the input signal and moderate impedance conversion.
• the second stage which buffers between the first stage and the load, further reduces output impedance while maintaining low power supply feedthrough.
• Power gain is achieved through the current amplification of the two stages with the beneficial aspects of the first stage achieved by a reduction in the geometry of the active device, and the beneficial aspects of the second stage achieved by a magnification of the geometry.
• this circuit provides electrical attenuation of the electro-acoustic transducer resonance, reducing the requirement for acoustic damping, a factor which, along with the ultra high impedance gate bias circuit, provides a significant improvement in the overall signal-to-noise ratio.
• the preamplifier of this invention reduces the noise level and therefore improves the signal-to-noise ratio of conventionally sized transducer devices. Furthermore, when this preamplifier is used with extremely small transducers, the smaller geometry of the first stage causes a lessened attenuation of the input signal and decreased power supply feedthrough due to capacitive coupling at the gate to the high side of the power supply.
• a feature of this invention resides in the reduced size of the input JFET as compared with the geometry hitherto employed in the above described prior art circuits, thereby reducing the capacitances between the gate and drain and between the gate and source, respectively. On the other hand, the size of the second stage JFET is substantially greater than the size hitherto normally employed in such prior art circuits.
• a further feature is the use of a resistor in series with parallel connected opposed diodes to form the gate bias for each of the first and second stage JFETs.
• This resistor acts to limit current if the amplifier is accidentally reverse biased.
• the effect of the diodes and protective resistor is to bias the gate above ground.
• the presently preferred embodiment of this invention is a preamplifier designated generally at 10, which is provided with input terminals 12 and 14 for connection to an electroacoustic transducer, for example an electret based condenser microphone used in hearing aids.
• an electroacoustic transducer for example an electret based condenser microphone used in hearing aids.
• Such transducers are characterized by a high output impedance and generally by some acoustic source noise, for example noise caused by acoustic resistive losses.
• One of these terminals is connected to the ground rail 16 of the circuit.
• An upper power supply rail 18 is connected to a terminal 20, and a dc source 22 which may be as low as 0.9 volt, for example, is connected between the terminal 20 and a terminal 24 which is connected to the rail 16.
• the output signal appears between the terminal 24 and a terminal 26.
• the circuit generally comprises a first stage 28 having a first JFET 30 connected in follower mode, and a second stage 32 having a second JFET 34 also connected in follower mode.
• the stages are interconnected by a shunt capacitor 36 and a coupling capacitor 38 the functions of which are respectively described below.
• the source, gate and drain terminals of the respective field effect transistors are correspondingly labeled "S,” “G” and “D,” respectively.
• the gate bias impedance circuit for the JFET 30 comprise a pair of parallel connected diodes 40 and 42 of opposite polarity and a resistor 44 connected in series therewith. The diodes act as resistors whose value is hundreds of giga-ohms.
• the gate bias impedance circuit for the JFET 34 comprises a pair of parallel connected diodes 46 and 48 of opposite polarity and a resistor 50 in series therewith.
• the load impedance of the circuit connected between the source of the JFET 34 and the ground rail 16 comprises a resistor 52.
• a resistor may be similarly connected between the source of the JFET 30 and ground, it is preferred to provide a third JFET 54 having its gate connected to ground and having its source connected through a resistor 56 to ground. Alternatively, its source may be connected directly to ground. As so connected the JFET 54 acts as a current source.
• a characteristic of the present invention is that the first stage or input JFET 30 is very much smaller in size than the output or second stage JFET 34.
• the size of a conventional transistor such as the 2N4338 manufactured by Siliconix, National Semiconductor, and others.
• the JFET 30 is less than half and preferably less than one-third the size of the conventional 2N4338, and the JFET 34 is on the order of two or more times the size of the conventional 2N4338 transistor.
• the ratio of sizes of the two JFETS is preferably six to one, and may be much greater in some cases.
• the size of the JFET 54 is typically smaller than that of the JFET 30.
• the pinch off voltage for the JFET 54 is equal to or lower than that for the JFET 30.
• the JFET 54 operates as the load by acting substantially as a current source, and limits the current to approximately one microampere. Because of the small size of the JFET 30 and the low current of the JFET 54, the output impedance of the first stage 28 is much higher than that found in conventional JFET circuits connected in follower mode, and the transconductance of the JFET 30 is lower.
• the capacitor 36 in shunt with the output impedance of the first stage, acts to roll off the higher frequencies.
• the capacitor 38 couples the signal to the gate of the JFET 34, and may be on the order of 50 pfd, but notwithstanding its small size it does not attenuate the low audio frequencies because the JFET 34 has a very high input impedance.
• the JFET 34 is larger than a normal transistor used in such circuits, so that the output impedance is low without excessive current draw.
• the size of the diodes employed in the circuit is conveniently compared with that of a conventional 1N4148 small signal diode manufactured by National Semiconductor, taken as a standard for reference.
• the diodes 40 and 42 are much smaller than the reference diode, for example one-tenth the size thereof, so as to minimize shunt capacitance and maximize shunt resistance.
• the value of the shunt capacitor 36 is chosen such that the combination of the first stage output impedance and the capacitor 36 forms an RC roll off at frequencies above a characteristic frequency f 0 .
• the latter frequency is chosen to coincide approximately with the incipient peaking of the acoustic circuit, resulting in a substantially flat overall response within the desired frequency pass band and an attenuation of the peak. Above the pass band, it results in an additional 6 dB per octave roll off beyond that which normally occurs from acoustic elements. The overall result is therefore a decrease in the total acoustic noise contribution of the source transducer.
• RC roll off has the further advantage that it can attenuate ultrasonic acoustic pickup and reduce the tendency for overload.
• circuits herein described may be constructed, in which the condenser 36 is replaced by an impedance network specifically designed to produce band acceptance, band rejection, high pass or low pass filtering, as dictated by the particular frequency shaping requirements of the application.
• impedance network specifically designed to produce band acceptance, band rejection, high pass or low pass filtering, as dictated by the particular frequency shaping requirements of the application.
• such networks may include inductive and resistive impedances or inductive, resistive and capacitive impedances.
• a further improvement may be made in the above-described circuit by employing a JFET 30 of asymmetric source and drain configuration, with the drain having fewer "fingers" than the source, with the result that when compared to a symmetric geometry the device will have a lower gate-to-drain capacitance and hence a lower power supply feedthrough.

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• Engineering & Computer Science (AREA)
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• Amplifiers (AREA)

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• 1995
  • 1995-05-23 US US08/447,349 patent/US5589799A/en not_active Expired - Fee Related
  • 1995-09-19 DE DE69506727T patent/DE69506727T2/de not_active Expired - Fee Related
  • 1995-09-19 EP EP95933142A patent/EP0783795B1/de not_active Expired - Lifetime
  • 1995-09-19 AU AU35913/95A patent/AU3591395A/en not_active Abandoned
  • 1995-09-19 DK DK95933142T patent/DK0783795T3/da active
  • 1995-09-19 WO PCT/US1995/011838 patent/WO1996010291A1/en active IP Right Grant

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Non-Patent Citations (1)

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